fix: avoid migrating slots in custom elements

Author: paoloricciuti

.changeset/eight-waves-mate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid migrating slots in custom elements

packages/svelte/src/compiler/migrate/index.js

@@ -583,6 +583,7 @@ const template = {
 		next();
 	},
 	SlotElement(node, { state, next }) {
+		if (state.analysis.custom_element) return;
 		let name = 'children';
 		let slot_name = 'default';
 		let slot_props = '{ ';

packages/svelte/tests/migrate/samples/slots-custom-element/input.svelte

@@ -0,0 +1,16 @@
+<svelte:options customElement="my-element" />
+
+<button><slot /></button>
+
+{#if foo}
+	<slot name="foo" {foo} />
+{/if}
+
+{#if $$slots.bar}
+	{$$slots}
+	<slot name="bar" />
+{/if}
+
+{#if $$slots.default}foo{/if}
+
+<slot name="dashed-name" />

packages/svelte/tests/migrate/samples/slots-custom-element/output.svelte

@@ -0,0 +1,16 @@
+<svelte:options customElement="my-element" />
+
+<button><slot /></button>
+
+{#if foo}
+	<slot name="foo" {foo} />
+{/if}
+
+{#if bar}
+	{$$slots}
+	<slot name="bar" />
+{/if}
+
+{#if children}foo{/if}
+
+<slot name="dashed-name" />